origin of the suffix -ikos


I was looking up the origin of the -ic syllable, as in democratic.

etymonline.com has:

Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames.
It seems the Greeks had -ikos.

Question: is this just a random suffix, or does come from some other word? Where might it have come from?

I tried looking for kos and ikos in a Greek dictionary and in Julius Pokorny's PIE dictionary, can't really find anything.

Any tips, links, guesses or wild speculations? Thanks!

Iohannes Aurum

Technicus Auxiliarius